1. Field of the Invention
The present invention relates to an electric double layer capacitor which includes a basic cell or a stack of basic cells each having a carbon paste polarized electrode sealed between a pair of collector electrodes and divided into two layers, the polarized electrode being compressed between the collector electrodes from outside thereof.
2. Description of the Prior Art
Motor vehicles powered by internal combustion engines, such as automobiles, have a starter motor for starting the engine, an alternator drivable by the engine for generating electric energy, and a lead storage battery for temporarily storing the electric energy generated by the alternator and for supplying the stored electric energy to the starter motor when the engine is to be started, or to other electric devices.
There has recently been developed a large-capacitance capacitor of the electric double layer type. Some such large-capacitance capacitors can be used as batteries. Japanese Patent Application No. 63(1988)-329846 discloses a motor vehicle power supply device in which such a large-capacitance capacitor is used to start an internal combustion engine.
FIG. 9 of the accompanying drawings shows an electric double layer capacitor having paste electrodes. The illustrated electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,936. The electric double layer capacitor comprises a single basic cell composed of a pair of current collectors 1 made of an electron conductor and which serves as a pair of collector electrodes, a pair of carbon electrodes 2 made of active carbon particles, a pair of nonconductive gaskets 3, and an isolating plate 4 for preventing electrons from moving between the electrodes 2.
The carbon electrodes 2 are made from a concentrated slurry which is a mixture of powdery or particulate active carbon and an electrolyte. The electrolyte has three functions to perform. It serves as an accelerator for ion conduction, an ion source, and a binder for carbon particles.
In order for an electric double layer capacitor to be used as a motor vehicle power supply, it has to have a large capacitance ranging from 100 to 150 F (farads), for example. If the necessary capacitance is to be achieved with an increased number of basic cells, then the capacitor becomes too heavy and bulky to be carried on the motor vehicle. It is necessary to increase the size of a basic cell in order to increase the energy density, i.e., the capacitance per unit volume or the capacitance per unit weight.
If the size of a basic cell is increased, the surface areas of the collector electrodes are also increased. Should different pressures be applied to the central and peripheral regions of the basic cell of increased size, a differential pressure between these different regions is developed. For an electric double layer capacitor to have stable performance as a power supply, it is necessary that the collector electrodes of the stacked basic cells be held in contact under uniform and sufficient adhesive forces.
The internal resistance of an electric double layer capacitor used as a motor vehicle power supply should be as low as possible. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes. Therefore, each basic cell should be kept under vertical pressure in order to reduce the internal resistance of the electric double layer capacitor. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2, though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used.
In ordinary electric double layer capacitors, the cells are kept under pressure by staking in order to reduce the internal resistance thereof. A self-supporting capacitor disclosed in Japanese Laid-Open Patent Publication No. 56(1981)-2621, for example, has an encased structure as shown in FIG. 10 of the accompanying drawings. More specifically, the self-supporting capacitor has an outer case 5 housing a first electrode plate 7 from which a first electrode terminal 6 extends vertically upwardly and a second electrode plate 10 from which a second electrode terminal 9 extends vertically upwardly, the second electrode plate 10 lying below the first electrode plate 7 with an insulating plate 8 interposed therebetween. With a certain number of basic cells 12 compressed by a reinforcing plate 11 and accommodated in the outer case 5, the upper peripheral edge of the outer case 5 is bent inwardly down against the first electrode plate 7 by staking near the electrode terminals 6, 9.
The electric double layer capacitor shown in FIG. 10 requires each cell to be kept under a pressure of ranging from 10 to 30 kg/cm.sup.2, though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. If the size of an electric double layer capacitor is so large that its surface area is 100 cm.sup.2 or more, then it may be kept under a pressure of several tons or higher.
When an electric double layer capacitor is compressed by staking, the pressure is also imposed on the gasket of each of the cells of the capacitor. Therefore, if the pressure applied to a electric double layer capacitor is to be increased, the thickness of the outer case has to be increased or the rigidity of the capacitor has to be increased by other methods. As a result, the prior electric double layer capacitors cannot be large because there is certain limitation on the overall weight and the cost of materials to be used. Furthermore, there are required a device for compressing the cells and also a device for retaining the cells by staking, and the process of applying a suitable pressure to the cells is complex.